1. Field of the Invention:
The present invention relates to a fixing apparatus of an image forming apparatus such as a copying machine, and a method of controlling temperature of the same.
2. Description of the Related Art:
Hereinafter, description is made regarding a method of controlling the temperature of a conventional fixing apparatus of a copying machine.
The fixing apparatus fixes a toner image transferred onto the surface of a copy paper, and is generally installed downstream in the copy paper conveying path. This fixing apparatus has a heat roller incorporating a heater and a press roller press-contacting the heat roller, and a copy paper passes between both rollers (hereinafter referred to as "paper passing operation"), and thereby the transferred toner image is heat-pressed to be fixed on the surface of the copy paper.
The heat roller has a heater, a cylindrical core metal encircling the heater, and a flexible elastic member having such as rubber which is affixed to the outer peripheral surface of this core metal. On the other hand, the press roller is formed with the same material as the above-mentioned core metal. In the heat roller, the temperature of the core metal is detected by a contact-type thermistor, and based on the result of the detection, the above-mentioned heater is controlled to keep the temperature of the core metal nearly constant. Thereby, the surface temperature of the elastic member is maintained also at a proper temperature (for example, 185.degree. C.).
Then, by the above-mentioned paper passing operation, heat of the surface of the elastic member is removed by the copy paper and dissipated. Intrinsically, an elastic member such as rubber has a low heat conductivity, and therefore heat from the heater in the heat roller cannot reach the surface thereof, and the surface temperature of the elastic member tends to be lower than the proper temperature. As a result, in the case of continuous copying, during the second and subsequent paper passing operations, fixing is performed at temperature (for example, 165.degree. C.) lower than the proper temperature, sometimes resulting in unsatisfactory fixing.
In addition, also during continuous copying, when heating of the above-mentioned internal heater of the heat roller is sufficient to keep the surface temperature of the elastic member at a proper value, problems are raised in that the adhesion between the core metal and the elastic member is affected or the elastic member deteriorates.
Then, the technique of compensating for a reduction in the surface temperature of the elastic member by an external heater in place of the internal heater has been proposed, for example, in the Japanese Patent Laid-Open No. Sho 54-29650 (29650/1979).
Then, FIG. 1 is a schematic cross-sectional view of the fixing apparatus thereof.
As described above, a heat roller 1 comprises an internal heater 5, a core metal 3, an elastic member 4, and a thermistor 8 detecting the temperature of the core metal 3. Numeral 2 designates the above-mentioned press roller, and numeral 6 designates a copy paper. Furthermore, a second thermistor 9 is installed on the surface of the elastic member 4 near the inlet of the copy paper passage to detect the surface temperature of that portion of the elastic member 4. Also, an external heating roller 15 incorporating a heater 7 contacts the elastic member 4 to heat the elastic member 4.
The fixing apparatus having such construction detects a reduction (165.degree. C.) in the surface temperature of the elastic member 4 due to paper passage by the above-mentioned second thermistor 9, and heats the member 4 by the external heating roller 15 to compensate for the reduction in temperature.
However, such a fixing apparatus has the following task to solve.
This means that during continuous paper passage, as described above, a large quantity of heat is taken away from the elastic member 4 by the copy papers, and therefore the external heating roller 15 continues heating to keep the elastic member 4 at a proper temperature (185.degree. C.). At this time, the surface temperature of the external roller 15 reaches a temperature as high as 240.degree. C. Now, when the paper passage ends, the heat dissipation thereof stops. Accordingly, a reduction in the temperature of the elastic member 4 does not occur.
However, the time required for a detection that heat dissipation has stopped, that is, the time required for detection of no reduction in temperature by the second thermistor 9 is the time required for the portion of elastic member, which has undergone heat dissipation, to rotate by a predetermined amount, pass through the position of contact with the external heating roller 15 and reach the place where the second thermistor 9 is located. Then, the heater 7 of the external heating roller 15 is turned off.
As a result, the temperature of the portion of the elastic member 4 undergoing no reduction in temperature, for example, 185.degree. C. at the outlet of paper passage, is further raised by the external heating roller 15 at 240.degree. C., and therefore far exceeds the allowable temperature of the elastic member 4, resulting in damage thereof.